Ground Penetrating Radar Resolution in Archaeological Geophysics

Abstract

Ground penetrating radar (GPR) is now a common tool for archaeological imaging. However, difficulties arise in choosing the right antenna. Do we choose high-frequency antennas to yield lots of detail? Or do we choose lower frequency antennas to see larger scale features that provide site context? Often we are tempted to use a high-frequency signal, in order to see all the detail. However, this can be counter-productive. Lots of detail may actually obscure the features that are the primary targets. Conversely, lower frequency antennas can locate the features of interest, but may not provide as much detail as desired. In general, choosing a lower frequency antenna yields better results. The optimum choice of antenna depends on the site conditions; using a range of antennas for initial tests helps establish the “best” signal frequency to use. The imaging may be best done in two stages: an initial stage using low frequency antennas; followed by high-frequency imaging to yield greater detail over the areas where it is useful.

In addition, conditions can change around a site, both laterally and with depth. For example, if there is a void—such as crypt beneath an old church site or a cave of archaeological interest—then the GPR velocity will be radically different across that void. If the void is air-filled, then the velocity will be much faster than in the surrounding soil or rock; any void reflections will arrive much sooner than those from within the surrounding material. If the void is water-filled, then the velocity will be much slower, causing a significant time delay when compared with the reflections from within the surrounding material. In addition, voids can generate multiple reflections because of the strong velocity contrast at the void top and bottom, and near-vertical voids, such as cracks, can generate stacks of diffractions caused by scattering from the rough walls of the crack.